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Southern Annular Mode

Today, glaciers in Patagonia and Antarctica are receding. In both cases, this has been attributed tochanges in the Southern Westerly Winds and the Southern Annular Mode (SAM). The SouthernWesterly Winds are an important wind belt that encircles the globe in the southern mid-latitudes1,2,with its core between 50-55°S1.

The Southern Annular Mode describes the north-south movement of this wind belt over a timescale ofdecades to centuries. The Southern Annular Mode is a key climatic component that will strongly affecthow glaciers in the Southern Hemisphere respond to climate change. It explains the key drivers forglaciation in the Southern Hemisphere, and why glacier advances are asynchronous with those in theNorthern Hemisphere3,4.

Positive phase of the SAM

The Southern Annular Mode is currently in a positive phase5, and this is projected to continue due toincreased greenhouse gas emissions. A positive phase of the Southern Annular Mode will continue todrive changes in the Southern Westerly Winds, causing warming and drying over Patagonia, andincreased upwelling of warm Circumpolar Deep Water and glacier recession in western Antarctica andthe Antarctic Peninsula.

The Southern Ocean climate systemThe Southern Westerly Winds and Southern Ocean are an important coupled climate system thatcontrols climate in the southern third of the world. This system is closely connected to CO2 degassingfrom the deep ocean, and the position of the Intertropical Convergence Zone1.

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Westerly Winds and ocean fronts around Antarctica

The Westerly Winds and Ocean Currents

The Southern Westerly Winds drive the Antarctic Circumpolar Current6. This current encirclesAntarctica. The northern boundary of the Antarctic Circumpolar Current is the Subantarctic Front(SAF), and the southerly boundary is the Polar Front (PF).

Antarctic Circumpolar Current

The Antarctic Circumpolar Current (ACC) is the World’s strongest current. It is able to flow unimpededthrough the Drake Passage and around the continent of Antarctica. The winds over the channel andthe flow of the ACC are aligned for the length of the channel. The ACC is getting stronger as theWesterly Winds have contracted poleward during the current postive phase of the Southern AnnularMode, as the winds are more directly aligned with the current26.

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Simplified schematic map of ocean currents of the Southern Ocean.

Circumpolar Deep Water

Beneath the cooler surface waters is a warmer layer of water. This is Circumpolar Deep Water (CDW),centered at a depth of ~500 m. It is dense and warm, with a high salt content27.

The Antarctic continental shelf is around 600 m deep, and deepens as it heads inland (a ‘retrograde’,or negative, slope). The ability of Circumpolar Deep Water to access the continental shelf isdetermined by the height of the CDW in the water column and the local bathymetry. The higher in thewater column the CDW sits, the more it will be able to cross the continental shelf27.

Strengthening of the regional westerly winds has increased the circulation of CDW onto thecontinental shelf, where it can reach ice-shelf cavities and grounding lines28. If more CDW canpenetrate onto the continental shelf, it may be able to reach the base of ice shelves on the Antarcticcontinent, melting them and risking ice-shelf collapse and marine ice sheet instability.

The Southern Annular ModeThe belt of the Southern Westerly Winds moves north and south over timescales of 10s to 100s ofyears. The Southern Annular Mode (SAM) (also known as the Antarctic Oscillation; AO) describes thenorth-south movement of these winds.

The Southern Annular Mode is usually defined as the difference in the zonal mean sea level pressureat 40°S (mid-latitudes) and 65°S (Antarctica)7,8. Changes in air pressure distribution causes changes in

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the strength and position of the westerly winds7. The SAM-index is effectively a measure of thestrength of the Southern Westerly Winds, and as it increases, the westerlies have been moving southand increasing27.

Positive Southern Annular Mode

In a Positive phase of the Southern Annular Mode, there is lower anomalous air pressure overAntarctica, and higher anomalous air pressure over the mid-latitudes7.

In a Positive Southern Annular Mode (the situation today), the belt of strong westerly windsstrengthens and contracts towards Antarctica. It weakens at the northern boundary in the mid-latitudes (40-50°S). It is drier over Patagonia, driving glacier recession. In Antarctica, increasedCircumpolar Deep Water upwells onto the continental shelf, driving glacier and ice sheet recession9.

Negative Southern Annular Mode

In a Negative Southern Annular Mode, the belt of strong Southern Westerly Winds expandsnorthwards towards the equator, bringing cold, wet weather to Patagonia and glacier advance, anddecreased Circumpolar Deep Water upwelling on the Antarctic Continental Shelf. The winds areweaker in this phase.

This was the situation during Holocene neoglaciations in Patagonia and during the Last GlacialMaximum4,10–12.

Positive and Negative phases of the Southern Annular Mode

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Changes in the Southern Annular Mode

We can track the past behaviour of the Southern Annular Mode in numerous proxies such as icecores8, lake sediment cores in Patagonia13 and glacier behaviour10. The Southern Westerly Winds inPatagonia can be reconstructed through changes in precipitation.

Over recent decades, the Southern Annular Mode has been consistently positive8. There have beendecreases in surface pressure over the Antarctic and corresponding increases in the mid-latitudes.This has resulted in an increase in the Westerlies over the Southern Ocean.

The positive trend in the Southern Annular Mode since ~AD 1940 has been attributed to risinggreenhouse gas levels and ozone depletion14, and the long-term average SAM index is now at itshighest level for the last 1000 years8.

Numerical models suggest that increased greenhouse gas emissions will drive further increases in theSAM-index over the next 100 years, driving further glacier recession in Patagonia and Antarctica8,15.Increasing CO2 leads to a poleward shift and a strengthening of the Southern Westerly Winds7.

Oceanographic implications of the increasing strength of the Westerlies ove rthe Southern Oceaninclude an intensification of the eddy field and a reduction in the efficiency of the Southern Ocean CO2

sink due to changes in upwelling and mixing23.

The Southern Westerly Winds in PatagoniaThe Southern Westerly Winds surround the Antarctic continent. The moisture within these windssustains the Patagonian icefields today, leading to large temperate ice masses16. At latitudes withinthe core of the wind belt, precipitation is high and temperature seasonality is low17.

The Andes mean that there is a strong precipitation gradient across Patagonia, with the westernAndes receiving vastly more precipitation than the drier lands to the east.

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Present-day climate in Patagonia (From Davies et al., 2020). A: mean annual air temperature. B: Meanannual precipitation. Note large precipitation gradient. C: Mean annual wind speed. D: Ocean frontsaround the Southern Hemisphere.

In Patagonia, changes in the Southern Westerly Winds have meant that it is warmer and drier. Thishas driven rapid glacier recession in Patagonia. Past periods of rapid glacier change in Patagonia havealso been attributed to changes in these winds. When the southern westerly wind belt moved north(for example, during the Antarctic Cold Reversal17), there was a strong precipitation increase inPatagonia, and the glaciers advanced. This has been related to negative phases of the SouthernAnnular Mode10,11.

Conversely, when the Southern Westerly Wind belt contracts, during positive phases of the SouthernAnnular Mode, it is warm and dry over Patagonia and glaciers shrink11. Over the last 10,000 years,several periods of glacier advance and retreat have been related to the changes in the SouthernAnnular Mode over a centennial timescale1.

The Southern Westerly Winds in AntarcticaA strengthening of the circumpolar vortex leads to a deepening of the Amundsen Sea Low, coolingmost of Antarctica but warming the Antarctic Peninsula, with drier conditions over West Antarcticaand the Ross Ice Shelf and Lambert Glacier23. The deepening of the Amundsen Sea Low, adjacent toThwaites Glacier, has influenced the amount of CDW beneath Thwaites Ice Shelf27.

The positive SAM index is driving surface cooling over East Antarctica. Winter cooling over EastAntarctica during periods with a high SAM index is due to the greater thermal isolation of Antarctica23.This is due to increased zonal flow, decreased meridional flow, and an intensified temperatureinversion on the ice sheet due to weaker near-surface winds23.

West Antarctica and Circumpolar Deep Water

Mass loss from Antarctica is dominated by the Amundsen Sea/Bellingshausen Sea sectors. In WestAntarctica, mass loss from West Antarctica has averaged 6.9 mm per decade from 1979-20179. Thismass loss is largely driven by ocean melt.

Stronger westerly winds in the Bellingshausen Sea and northern Amundsen Sea have changed oceancirculation, leading to more Circumpolar Deep Water (CDW) intruding onto the continental shelf18,towards the grounding zones of Thwaites Glacier19. The enhanced polar westerlies are pushing moreCircumpolar Deep Water onto the continental shelf where it melts the base of ice shelves9,20. Continental shelf waters in the Amundsen Sea have warmed, causing ice shelf thinning and retreat ofgrounding lines21.

The mass loss in the Amundsen Sea, Wilkes, Western Peninsula and Bellingshausen Sea sectors ofAntarctica has been increasing since the 1970s, and this is consistent with the polar contraction of thewesterlies that force more CDW onto the continental shelf9. The Circumpolar Deep Water reachesglaciers through the deep bathymetric troughs on the sea floor that were carved by former icestreams. This water can then melt ice shelves and destabilise glaciers9.

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Warm Circumpolar Deep Water is penetrating beneath the ice shelves of Pine Island Glacier andThwaites Glacier.

Antarctic Peninsula

Around the Antarctic Peninsula, glacier advances and recessions have been related to a weakening orstrengthening of the band of Southern Westerly Winds10. Around the Western Antarctic Peninsula,glaciers that terminate in warm Circumpolar Deep Waster have undergone considerable retreat 22.

The positive phase of the SAM also causes warming on the Antarctic Peninsula8. The greatest warminghas been observed during the summer months, and is associated with the strengthening of theSouthern Westerly Winds and the positive phase of the Southern Annular Mode. Stronger winds haveresulted in more warm, maritime air masses crossing the peninsula and reaching the low-lying iceshelves. The spine of the Antarctic Peninsula mountains also causes the adiabatic descent andwarming of the winds as they cross the topography23 .

Increased sea ice and an equatorward expansion of the Westerlies favour larger glaciers in Patagoniaand the eastern Antarctic Peninsula. Reduced glacier extents in Patagonia and the eastern AntarcticPeninsula occur when conditions resemble a persistent positive Southern Annular Mode10.

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Ice cores on the Antarctic Peninsula, on the Landsat Image Mosaic of Antarctica.

Future projectionsUnder a limited warming scenario (~1.5°C), with the gradual repair of the ozone hole and stabilisationof greenhouse gas emissions, it is likely that the Southern Westerly Winds will return to values typicalof the Twentieth Century24. Ozone recovery in particular will encourage a weakening of thecircumpolar westerlies25.

Under a higher emissions scenario, increases in greenhouse gases would cause a continued shift inthe Southern Westerly Winds poleward, with a strengthening in all seasons24. Wind-driven changes inocean currents will increase ocean heat transport to the Amundsen Sea, entering the cavities beneathfloating ice shelves and driving higher basal melt, and a reduction in backstress on grounded iceupstream24.

Further readingNCAR Climate Data Guide: Marshall Southern Annular Mode (SAM) indexCircumpolar Deep WaterWesterly Winds

References

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